Control device for fuel cell vehicle

ABSTRACT

The present invention intends to provide a control device for a fuel cell vehicle which suppresses the deterioration of driveability upon acceleration without increasing the size of a driving electric motor or a motor driver. When a requested output (PD_CAL) exceeds a first output limit determined depending on a continuous output rating of an electric motor  10  or a motor driver  5  and an increasing rate of the requested output (PD_CAL) exceeds a reference increasing rate, an upper-limit target output determining unit  52  determines an assist time used as a time during which the electric motor  10  can be operated continuously by the requested output (PD_CAL) on the basis of the requested output (PD CAL) and an open-circuit voltage (Vcap_o) of a capacitor  3.  Until the assist time is elapsed, the upper-limit target output (PD_LMT) is set to a second output limit (&gt;the first output limit) determined depending on a short-time output rating of the electric motor  10  or the motor driver  5.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a control device for a fuel cellvehicle using a fuel cell and an electric double layer capacitor, whichare connected parallel to each other, as a power supply for a drivingelectric motor. Particularly, the invention relates to a control of anoutput of the electric motor.

[0003] 2. Description of the Related Art

[0004] As a control device, mounted on a fuel cell vehicle, forcontrolling a driving electric energy supplied to a driving electricmotor for the fuel cell vehicle, for example, a control deviceconstituted as shown in FIG. 6 is known.

[0005] The control device shown in FIG. 6 has a motor driver 101 forsupplying a driving electric energy to a driving electric motor 100 anda fuel cell 102 functioning as a power supply for an electric accessorysuch as an air-conditioner (not shown). The control device isconstituted in the following manner. An electric double layer capacitor103 is connected parallel to the fuel cell 102. If a response from areactive gas supply unit 104 such as an air-conditioning compressor forsupplying reactive gases to the fuel cell 102 is delayed and thereactive gases supplied to the fuel cell 102 are insufficient to cause ashortage of the amount of electric energy generated by the fuel cell102, the shortage is compensated with an electric energy discharged fromthe electric double layer capacitor 103.

[0006] The motor driver 101 supplies a driving electric energy dependingon a torque command (TRQ_CMD) which is generated from an electricvehicle control unit 105 to the electric motor 100. The reactive gassupply unit 104 regulates the rate at which the reactive gases aresupplied to the fuel cell 102 so as to obtain the amount of generatedelectric energy depending on a target output (PD_REQ) of the electricmotor 100, the target output being calculated by a target outputcalculator 106.

[0007] In this instance, the target output calculator 106 calculates thetarget output (PD REQ) of the electric motor 100 fundamentally on thebasis of a requested output (PD_CAL) of the electric motor 100determined depending on the rotational speed (Nm) of the electric motorand the amount of depression (Ap) of an accelerator pedal.

[0008] However, when the requested output (PD_CAL) exceeds a continuousoutput rating (PD_LMT) of the motor driver 101 or the electric motor100, the target output calculator 106 limits the target output (PD_REQ)to the continuous output rating (PD_LMT) or lower to calculate thetarget output, thereby preventing the motor driver 101 or the electricmotor 100 from running over the continuous output rating (PD_LMT). Atorque command calculator 107 calculates the torque command (TRQ_CMD)for the motor driver 101 depending on the target output (PD_REQ).Consequently, the output of the electric motor 100 is suppressed to thecontinuous output rating (PD_LMT) or lower.

[0009] Therefore, when the driver of the fuel cell vehicle depresses theaccelerator pedal in order to accelerate the fuel cell vehicle, theoutput of the electric motor 100 is suppressed to the continuous outputrating or lower and a sensation of acceleration that the driver desiresis not derived. The driveability may deteriorate. In order to preventthe driveability from deteriorating, the adoption of the motor driver101 or the electric motor 100 with a higher continuous output rating isconsidered. In this case, there are the following disadvantages. Sincethe size of the motor driver 101 or the electric motor 100 is increased,a space for installation therefor is also increased. The cost of themotor driver 101 or the electric motor 100 also increases.

SUMMARY OF THE INVENTION

[0010] The present invention is made in order to solve the abovedisadvantages. It is an object of the present invention to provide acontrol device for a fuel cell vehicle which suppresses thedeterioration of driveability upon acceleration without increasing thesize of a driving electric motor or a motor driver.

[0011] According to the present invention, there is provided a controldevice for a fuel cell vehicle, comprising motor-requested-outputdetermining means for determining a requested output for a drivingelectric motor, motor-target-output calculating means for calculating atarget output of the electric motor while limiting the target output toa predetermined upper-limit target output or lower depending on therequested output, motor driving means for outputting a driving electricenergy depending on the target output to the electric motor, a fuel cellwhich is used as a power supply for the motor driving means, an electricdouble layer capacitor which is connected parallel to the fuel cell soas to be charged by the fuel cell and to be discharged for compensationfor the insufficient amount of generated electric energy when the amountof electric energy generated by the fuel cell is insufficient, reactivegas supply means for supplying reactive gases at a rate depending on thetarget output to the fuel cell.

[0012] The control device further includes capacitor-charged-amountrecognizing means for recognizing the amount of electric energy chargedinto the electric double layer capacitor, and assist-time calculatingmeans for calculating an assist time used as a time during which theelectric motor can be operated continuously by the requested output onthe basis of the requested output and the amount of electric energycharged into the electric double layer capacitor in an abruptlyincreasing state of the requested output in which the requested outputexceeds a first output limit and an increasing rate of the requestedoutput exceeds a predetermined reference increasing rate, the firstoutput limit being determined in accordance with a continuous outputrating of the electric motor or the motor driving means. When therequested output is not in the abruptly increasing state, themotor-target-output calculating means sets the first output limit to theupper-limit target output, and when the requested output is in theabruptly increasing state, the motor-target-output calculating meanssets a second output limit larger than the first output limit to theupper-limit target output within the assist time, the second outputlimit being determined in accordance with a short-time output rating ofthe motor driving means or the electric motor.

[0013] According to the present invention, the continuous output ratingmeans an output which can be continuously generated from the electricmotor or an electric energy which can be continuously generated from themotor driving means to the electric motor. The smaller one of the outputand the electric energy specifies the maximum output which can becontinuously extracted from the electric motor.

[0014] The short-time output rating means an output which can beobtained from the electric motor within a first fixed time or anelectric energy which can be generated from the motor driving means tothe electric motor within a second fixed time. An output which can beobtained from the electric motor within a short time (the first fixedtime or the second fixed time) is determined in accordance with thesmaller one of the output and the electric energy.

[0015] When the requested output is in the abruptly increasing state,the motor-target-output calculating means sets the second output limitlarger than the first output limit within the assist time. Therefore,after the abruptly increasing state of the requested output, an outputexceeding the first output limit can be generated from the electricmotor by the electric energy discharged from the capacitor within theassist time. Consequently, in the abruptly increasing state of therequested output, the deterioration of driveability caused by a shortageof the output of the electric motor can be suppressed.

[0016] When the upper-limit target output is returned from the secondoutput limit to the first output limit immediately after the assisttime, the target output calculated by the motor-target-outputcalculating means decreases abruptly and the output of the electricmotor is also lowered abruptly in association with the abrupt decrease.As a result, the motion of the fuel cell vehicle becomes unstable.

[0017] Accordingly, the motor-target-output calculating means reducesthe upper-limit target output to the first output limit at apredetermined decreasing rate after the assist time has passed.Consequently, the output of the electric motor is prevented fromabruptly decreasing after the assist time and the upper-limit targetoutput can be returned to the first output limit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a block diagram showing the configuration of a controldevice for a fuel cell vehicle according to the present invention;

[0019]FIG. 2 is a block diagram showing the control arrangement of thecontrol device shown in FIG. 1;

[0020]FIG. 3 is a flowchart showing the operation of a driver controlunit;

[0021]FIGS. 4A and 4B are graphs to determine an assist time;

[0022]FIG. 5 includes time series graphs (1) to (3) showing theoperation and the state of the control device during the assist time andbefore and after the assist time; and

[0023]FIG. 6 is a block diagram showing the configuration of aconventional control device for a fuel cell vehicle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] An embodiment of the present invention will now be described withreference to FIGS. 1 to 5. FIG. 1 is a block diagram showing theconfiguration of a control device for a fuel cell vehicle according tothe present invention, FIG. 2 is a block diagram showing the controlarrangement of the control device shown in FIG. 1, FIG. 3 is a flowchartshowing the operation of a driver control unit shown in FIG. 2, FIGS. 4Aand 4B are correlation graphs to determine an assist time, and FIG. 5includes time series graphs showing the operation and the state of thecontrol device during the assist time and before and after the assisttime.

[0025] Referring to FIG. 1, a control device 1 for a fuel cell vehicle(hereinbelow, simply referred to as a control device 1) according to thepresent invention is mounted on a fuel cell vehicle and controls anelectric energy supplied to the fuel cell vehicle. A controller 4controls the operation of the control device 1. The controller 4comprises a microcomputer, a memory, and other components.

[0026] A fuel cell 2 outputs an electric current based on anelectrochemical reaction between reactive gases of hydrogen and air. Theamount of electric energy generated by the fuel cell 2 is controlled bya power supply management control unit 14 and a fuel cell control unit16 which are provided for the controller 4. A driver control unit 9provided for the controller 4 determines a torque command to a drivingelectric motor 10 (hereinbelow, simply referred to as an electric motor10).

[0027] An output electric energy produced by the fuel cell 2 and anelectric double layer capacitor 3 (hereinbelow, simply referred to as acapacitor 3) is supplied to a motor driver 5 (corresponding to motordriving means according to the present invention), an air-conditioningunit 6, and a 12-V load 8 through a DC/DC converter 7. The motor driver5 controls currents flowing through armatures of the electric motor 10depending on a torque command (TRQ_CMD) outputted from the drivercontrol unit 9. A drive power generated by the electric motor 10 istransferred to drive wheels 12 through a transmission 11.

[0028] The driver control unit 9 outputs a signal indicative of a targetoutput (PD_REQ) of the electric motor 10, the output being calculatedbased on the amount of depression (Ap) of an accelerator pedal 13 andthe rotational speed (Nm) of the electric motor 10, to the power supplymanagement control unit 14.

[0029] The power supply management control unit 14 is supplied withdetected signals of a load current (Iload) and a load voltage (Vload)which are detected by a load sensor 15 in order to recognize theelectric energy consumed by electric accessories other than the electricmotor 10. The power supply management control unit 14 recognizes theelectric energy consumed by the electric accessories other than theelectric motor 10 on the basis of the detected signals.

[0030] In consideration of an upper limit amount of generated electricenergy (Ifc_LMT) outputted from the fuel cell control unit 16 andindicative of an upper limit amount of electric energy generated by thefuel cell 2, a current (Icap) charged into and discharged from thecapacitor 3, and a voltage (Vcap) across the capacitor 3, the current(Icap) and the voltage (Vcap) being detected by a capacitor sensor 31,the power supply management control unit 14 determines a target amountof generated electric energy (Ifc_CMD) which is a target value for acurrent outputted from the fuel cell 2 depending on the sum of thetarget output (PD_REQ) of the electric motor 10 and the electric energyconsumed by the electric accessories other than the electric motor 10,and then outputs a signal indicative of the target amount of generatedelectric energy (Ifc_CMD) to the fuel cell control unit 16.

[0031] The power supply management control unit 14 outputs a signalindicative of an output limit electric energy (PLD) representing anupper limit of the electric energy that can be supplied from the fuelcell 2 and the capacitor 3 to the motor driver 5, to the driver controlunit 9.

[0032] The detected signals outputted from a reactive gas sensor 20 andindicating a pressure (Pgas), a flow rate (Qgas), and a temperature(Tgas) of reactive gases (hydrogen and air) supplied to the fuel cell 2,and detected signals indicative of states (Vcell_indiv) of individualfuel cells (not shown) that make up the fuel cell stack are inputted tothe fuel cell control unit 16. The fuel cell control unit 16 determinesthe upper limit amount of generated electric energy (Ifc LMT) inconsideration of the state of the fuel cell 2 as recognized from thesedetected signals.

[0033] The driver control unit 9 outputs the torque command (TRQ_CMD) tothe motor driver 5 so that the electric energy consumed by the electricmotor 10 and the motor driver 5 does not exceed the output limitelectric energy (PLD) indicated by the power supply management controlunit 14. The motor driver 5 controls the armature currents of theelectric motor 10 to cause the electric motor 10 to generate a torquedepending on the torque command (TRQ_CMD).

[0034] The fuel cell control unit 16 outputs a signal indicative of atarget amount of reactive gases (CMP_CMD) supplied to the fuel cell 2 toa reactive gas supply device 21 (corresponding to reactive gas supplymeans according to the present invention) so that the fuel cell 2 willoutput a current for the target amount of generated electric energy(Ifc_CMD) outputted from the power supply management control unit 14.Consequently, the fuel cell 2 is supplied with air and hydrogen at arate depending on the target amount of generated electric energy(Ifc_CMD).

[0035] Hydrogen supplied from the reactive gas supply device 21 issupplied to hydrogen electrodes of the fuel cell 2 through an ejector(not shown) and a humidifier (not shown), reacts electrically andchemically with oxygen in air supplied to air electrodes of the fuelcell 2, producing water which is discharged through a discharge valve22. The opening of the discharge valve 22 is controlled by a controlsignal (VLV_CMD) supplied from the fuel cell control unit 16 in order tokeep the pressure in the fuel cell 2 at a constant gradient depending onthe pressures of the supplied air and hydrogen.

[0036] The fuel cell 2 has a water-cooled cooling unit (not shown). Thefuel cell control unit 16 controls the rate and temperature of coolingwater supplied to the water-cooled cooling unit depending on thetemperature of the cooling water supplied to the water-cooled coolingunit and the temperature of the cooling water discharged from thewater-cooled cooling unit.

[0037] The control device 1 also has output current limiting means 30(including a function as current limiting means according to the presentinvention) which has a switching device such as a transistor or an FETin order to limit a current outputted from the fuel cell 2 and detectsan output current (Ifc) and an output voltage (Vfc) of the fuel cell 2.The output current limiting means 30 turns on or off the currentoutputted from the fuel cell 2 depending on a level (high/low) of acurrent limit signal (VCU_CMD) outputted from the power supplymanagement control unit 14.

[0038] Fundamentally excepting the start time and the stop time of thefuel cell 2, the power supply management control unit 14 always sets thecurrent limit signal (VCU_CMD) at a high level to turn on (energize) theoutput current limiting means, thereby keeping the direct coupling stateof the fuel cell 2 and the capacitor 3.

[0039] In the direct coupling state, when the total amount of electricenergy consumed by the electric motor 10 and the electric accessoriesother than the electric motor 10 increases, resulting in the decrease ofthe output voltage of the fuel cell 2, a discharge current depending ona difference between an open-circuit voltage of the capacitor 3 and theoutput voltage of the fuel cell 2 is supplied to the electric motor 10and the electric accessories other than the electric motor 10. On theother hand, when the total amount of consumed electric energy isreduced, resulting in the increase of the output voltage of the fuelcell 2, the charged current depending on the difference between theopen-circuit voltage of the capacitor 3 and the output voltage of thefuel cell 2 is supplied from the fuel cell 2 to the capacitor 3.

[0040] Consequently, in both the cases, the open-circuit voltage of thecapacitor 3 becomes equivalent to the output voltage of the fuel cell 2.Therefore, it is unnecessary to always cause the output voltage of thefuel cell 2 to match an open-circuit voltage of a battery by a largeDC/DC converter which can switch a heavy current as in the case wherethe battery is connected parallel to the fuel cell 2. In such a battery,the open-circuit voltage does not change very much even if the remainingamount of charged electric energy changes.

[0041] Accordingly, the output current limiting means 30 may have asmall switching device to limit the passage of the electric currentbetween the capacitor 3 and the fuel cell 2 at the start time and thestop time of the fuel cell 2, during which the output current of thefuel cell 2 is small.

[0042] With the constitution described above, the target amount ofreactive gases (CMP_CMD) is controlled so that the fuel cell 2 outputs acurrent depending on the target amount of generated electric energy(Ifc_CMD) determined based on the electric energy consumed by theelectric accessories calculated on the basis of the target output(PD_REQ), the load current (Iload), and the load voltage (Vload) of theelectric motor 10.

[0043] The driver control unit 9 limits the target output (PD_REQ) ofthe electric motor 10 so that each of the motor driver 5 and theelectric motor 10 does not operate over the rating. The output of theelectric motor 10 is lowered in association with the above limiting,surpressing the deterioration of driveability of the fuel cell vehicle.A process of calculating the target output (PD_REQ) of the electricmotor 10 by the driver control unit 9 will now be described withreference to FIGS. 2 to 5.

[0044] Referring to FIG. 2, the driver control unit 9 comprises arequested-output calculator 50 (corresponding to motor-requested-outputdetermining means according to the present invention), a target outputcalculator 51 (including a function as motor-target-output calculatingmeans according to the present invention), an upper-limit target outputdetermining unit 52 (including functions as assist time calculatingmeans and motor-target-output calculating means according to the presentinvention), and a target output limiting unit 53.

[0045] The requested-output calculator 50 calculates a requested output(PD_CAL) of the electric motor 10 on the basis of the amount ofdepression (Ap) of the accelerator pedal 13 (refer to FIG. 1) and therotational speed (Nm) of the electric motor 10.

[0046] The target output calculator 51 calculates the target output(PD_REQ) of the electric motor 10 depending on the requested output(PD_CAL) so as not to exceed an upper-limit target output (PD_LMT)determined by the upper-limit target output determining unit 52.

[0047] The upper-limit target output determining unit 52 usuallydetermines a first output limit (L1), which is determined depending onthe continuous output rating of the motor driver 5 or the electric motor10, as the upper-limit target output (PD_LMT). When the requested output(PD_CAL) is increased abruptly, the upper-limit target outputdetermining unit 52 determines a second output limit (L₂), which isdetermined depending on the short-time output rating of the motor driver5 or the electric motor 10, as the upper-limit target output (PD_LMT).

[0048] The target output limiting unit 53 fundamentally determines thetorque command (TRQ_CMD) so as to obtain the target output (PD_REQ) fromthe electric motor 10. When the electric energy needed for the motordriver 5 exceeds the output limit electric energy (PLD) in order toobtain the target output (PD_REQ), the target output limiting unit 53limits the torque command (TRQ_CMD) so that the electric energy consumedby the motor driver 5 is lower than the output limit electric energy(PLD).

[0049] In this instance, in consideration of the electric energydischarged from the capacitor 3, the power supply management controlunit 14 calculates the output limit electric energy (PLD). In otherwords, first, an open-circuit voltage calculator 60 (including afunction as capacitor-charged-amount recognizing means according to thepresent invention) calculates an open-circuit voltage (Vcap_o) of thecapacitor 3 on the basis of data indicating an internal resistance(Rcap) of the capacitor 3 stored in a memory, a capacitor voltage(Vcap), and a capacitor current (Icap) using the following equation (1).

Vcap _(—) o=Vcap+Icap×Rcap  (1)

[0050] When the output voltage of the capacitor 3, namely, the amount ofelectric energy generated by the fuel cell 2 obtained by applying theupper limit amount of generated electric energy (Ifc_LMT) to electriccurrent/voltage output characteristic map of the fuel cell 2, stored ina memory, is equivalent to the upper limit amount of generated electricenergy (Ifc_LMT), a capacitor-discharged-electric-energy calculator 61calculates an upper-limit discharged electric energy (Pcap_LMT) as anelectric energy generated from the capacitor 3 using the followingequation (2).

Pcap _(—) LMT=(Vcap _(—) o−Vfc _(—) LMT)/Rcap×Vfc _(—) LMT  (2)

[0051] A motor-drive-upper-limit-electric-energy calculator 63 subtractsthe electric energy (=V_load×i_load) consumed by the electricaccessories from the sum of the output electric energy of the fuel cell2 depending on the upper-limit amount of generated electric energy(Ifc_LMT) and the upper-limit electric energy (Pcap_LMT) discharged fromthe capacitor 3, thereby calculating the output limit electric energy(PLD). Accordingly, the output limit electric energy (PLD) is set inconsideration of the electric energy discharged from the capacitor 3.

[0052] The operation of the upper-limit target output determining unit52 will now be described with reference to the flowchart shown in FIG.3. When the operation of the driver control unit 9 is started byenergizing the controller 4, the upper-limit target output determiningunit 52 resets a flag (flag=0) as initialization in STEP 1 and alsodetermines the foregoing first output limit (L₁) as the upper-limittarget output (PD_LMT).

[0053] The subsequent STEP 2 indicates a process by the requested-outputcalculator 50. As mentioned above, the requested-output calculator 50calculates the requested output (PD_CAL) of the electric motor 10 on thebasis of the amount of depression (Ap) of the accelerator pedal and thelike.

[0054] The upper-limit target output determining unit 52 checks whetherthe flag has been set in the next STEP 3. If the flag has not been set(flag=0), STEP 4 follows. The upper-limit target output determining unit52 checks whether the rate of increase of the requested output (PD_CAL)exceeds the reference rate of increase.

[0055] If the increasing rate of the requested output (PD_CAL) exceedsthe reference increasing rate, the process branches to STEP 10. When therequested output (PD_CAL) is lower than the first output limit (L₁), theprocess proceeds to STEP 5 and the upper-limit target output determiningunit 52 resets the flag. The first output limit (L₁) is set to theupper-limit target output (PD_LMT) in STEP 6. If the operation is beingcontinued in STEP 7, the process returns to STEP 2.

[0056] Accordingly, in the case where the increasing rate of the amountof depression (Ap) of the accelerator pedal is equal to or lower thanthe reference increasing rate and the driver of the fuel cell vehicledoes not desire abrupt acceleration, such a loop of STEPS 2 to 7 isrepetitively executed to keep the upper-limit target output (PD_LMT) atthe first output limit (L₁).

[0057] On the other hand, in the case where the increasing rate of theamount of depression (Ap) of the accelerator pedal exceeds the referenceincreasing rate in STEP 4, the process branches to STEP 10, and therequested output (PD_CAL) exceeds the first output limit (L₁) in STEP 10(such a state corresponds to the abruptly increasing state of therequested output according to the present invention), the processproceeds from STEP 10 to STEP 11. In this case, since the flag is notyet set, the process proceeds to STEP 12.

[0058] In STEP 12, the upper-limit target output determining unit 52determines an assist time used as a time during which the electric motor10 can be driven at the target output (PD_REQ) exceeding the firstoutput limit (L₁). The upper-limit target output determining unit 52determines the assist time on the basis of a correlation graph of motorassist time/motor requested output (PD_CAL) shown in FIG. 4A and acorrelation graph of capacitor assist time/capacitor open-circuitvoltage (Vcap_o) shown in FIG. 4B. Data items of these correlationgraphs have been stored in the memory.

[0059] In the correlation graph of FIG. 4A, the axis of ordinate (t)denotes the motor assist time and the axis of abscissa (W) denotes therequested output (PD_CAL) of the electric motor 10. The motor assisttime indicates a time during which the electric motor 10 can be drivencontinuously at the requested output (PD_CAL). As the requested output(PD_CAL) of the electric motor 10 becomes higher, the motor assist timebecomes shorter.

[0060] In the correlation graph of FIG. 4B, the axis of ordinate (t)denotes the capacitor assist time and the axis of abscissa (V) denotesthe open-circuit voltage (Vcap_o) of the capacitor 3. The capacitorassist time denotes a time during which a supplementary electric energycan be generated from the capacitor 3 in order to generate an electricenergy larger than the first output limit (L₁) from the capacitor 3 tothe motor driver 5. As the open-circuit voltage (Vcap_o, which varies inproportion to the amount of electric energy charged into the capacitor3) is larger, namely, as the amount of electric energy charged into thecapacitor 3 is larger, the capacitor assist time is longer.

[0061] The upper-limit target output determining unit 52 compares themotor assist time obtained by applying the requested output (PD_CAL) tothe correlation graph of FIG. 4A with the capacitor assist time obtainedby applying the open-circuit voltage (Vcap_o) of the capacitor 3,calculated by the open-circuit voltage calculator 60, to the correlationgraph of FIG. 4B, and determines the time shorter than the other time asthe assist time.

[0062] After the assist time is determined in this manner, theupper-limit target output determining unit 52 sets a flag (flag=1) inSTEP 13, a timer which uses the assist time as a set time is started inSTEP 14, the upper-limit target output (PD_LMT) is set to the secondoutput limit L₂ (>L₁) in STEP 15, and the process proceeds to STEP 7.

[0063] Consequently, the target output calculator 51, which haspreviously calculated the target output (PD_REQ) of the electric motor10 on the basis of the first output limit (L₁) as the upper limit,calculates the target output (PD_REQ) using the second output limit (L₂)as the upper limit. Therefore, the output of the electric motor 10 canbe increased in accordance with the driver's intention of abruptlyaccelerating the fuel cell vehicle.

[0064] While the requested output (PD_CAL) exceeds the first outputlimit (L₁) in STEP 10, the process branches from STEP 11 to STEP 20.Until the time set by the timer is up, namely, the assist time iselapsed, the process branches from STEP 20 to STEP 7. Accordingly, theupper-limit target output (PD_LMT) is held at the second output limit(L₂), so that the output of the electric motor 10 can be higher than thefirst output limit (L1).

[0065] When the time set by the timer is up in STEP 20, the processproceeds to STEP 21. The upper-limit target output determining unit 52performs a process of reducing the upper-limit target output (PD_LMT)stepwisely by a predetermined decreasing amount (ΔPD) to reduce theupper-limit target output (PD_LMT) at a predetermined rate of decrease.In other words, in STEP 21, when subtracting the decreasing amount (ΔPD)from the current upper-limit target output (PD_LMT), the upper-limitoutput determining unit 52 checks whether the upper-limit target outputis equal to or lower than the second output limit (L₂) (PD_LMT−ΔPD≦L₂).

[0066] If NO (PD_LMT−ΔPD>L₂), the process proceeds to STEP 22 and thedecreasing amount (ΔPD) is subtracted from the current upper-limittarget output to set the new upper-limit target output (PD_LMT).Consequently, the upper-limit target output (PD_LMT) is reducedstepwisely as much as the decreasing amount (ΔPD) until PD_LMT−ΔPD≦L₂ inSTEP 21.

[0067] If PD_LMT−ΔPD≦L₂ in STEP 21, the process branches to STEP 30. Theupper-limit target output determining unit 52 resets a flag (flag=0) andthen returns the upper-limit target output (PD_LMT) to the first outputlimit (L₁) in the next STEP 31.

[0068] As mentioned above, after the assist time is elapsed, theupper-limit target output (PD_LMT) is gradually returned to the firstoutput limit (L₁) at the predetermined decreasing rate. Consequently,the output of the electric motor 10 can be prevented from abruptlydecreasing after the assist time.

[0069] According to the present embodiment, the upper-limit targetoutput (PD_LMT) is set to the second output limit (L₁) until the assisttime is elapsed. When a time during which the upper-limit target output(PD_LMT) is set to the second output limit (L₂) is determined within theassist time, the advantages of the present invention can be obtained.

[0070]FIG. 5 includes the graphs (1) to (3) showing the change of theoutput of the electric motor 10 in the case where the upper-limit targetoutput (PD_LMT) is changed from the first output limit (L₁) to thesecond output limit (L₂) only within the assist time as mentioned above.The graph (1) shows the change of the requested output (PD_CAL), thegraph (2) shows the change of an output (Pmot) of the electric motor 10,and the graph (3) shows the change of the open-circuit voltage (Vcap_o)of the capacitor 3.

[0071] In the graph (1), at time t₁, the requested output (PD_CAL) at alevel A₁ starts to abruptly increase at an increasing rate higher thanthe reference increasing rate. When the requested output exceeds thefirst output limit (L₁) at time t₂, the upper-limit target outputdetermining unit 52 switches the upper-limit target output (PD_LMT)which has previously been set to the first output limit (L₁) to thesecond output limit (L₂).

[0072] Consequently, as shown in the graph (2), the output of theelectric motor 10 exceeds the first output limit (L₁) for a period untilan assist time (AS_TIME) is elapsed after time t₂. The upper-limittarget output determining unit 52 reduces the upper-limit target output(PD_LMT) at the predetermined decreasing rate after time t₃ at which theassist time (AS_TIME) is elapsed up to time t₄ at which the output isset to the first output limit (L₁). Accordingly, as shown in the graph(2), the output of the electric motor 10 decreases gradually after timet₃ to time t₄.

[0073] The electric energy discharged from the capacitor 3 is consumedby the motor driver 5 for the output of the electric motor 10 exceedingthe first output limit (L₁) from time t₂ to time t₄. Accordingly, asshown in the graph (3), the open-circuit voltage (Vcap_o) of thecapacitor 3 decreases after time t₂.

[0074] According to the present embodiment, the upper-limit targetoutput determining unit 52 performs the process of reducing theupper-limit target output (PD_LMT) at the predetermined decreasing rateafter the assist time. The advantages of the present invention areeffective even if such a process is not performed.

[0075] According to the present embodiment, the requested-outputcalculator 50 calculates the requested output (PD_CAL) of the electricmotor 10 on the basis of two parameters, namely, the amount ofdepression (Ap) of the accelerator pedal 13 and the rotational speed(Nm) of the electric motor 10. The requested output (PD_CAL) of theelectric motor 10 may be calculated on the basis of any one of theparameters, for example, only the amount of depression (Ap) of theaccelerator pedal 13.

[0076] Although a certain preferred embodiment of the present inventionhas been shown and described in detail, it should be understood thatvarious changes and modifications may be made therein without departingfrom the scope of the appended claims.

What is claimed is:
 1. A control device for a fuel cell vehicle,comprising: motor-requested-output determining means for determining arequested output for a driving electric motor; motor-target-outputcalculating means for calculating a target output of the electric motorwhile limiting the target output to a predetermined upper-limit targetoutput or lower depending on the requested output; motor driving meansfor outputting a driving electric energy depending on the target outputto the electric motor; a fuel cell which is used as a power supply forthe motor driving means; an electric double layer capacitor which isconnected parallel to the fuel cell so as to be charged by the fuel celland to be discharged for compensation for the insufficient amount ofgenerated electric energy when the amount of electric energy generatedby the fuel cell is insufficient; reactive gas supply means forsupplying reactive gases at a rate depending on the target output to thefuel cell; capacitor-charging-amount recognizing means for recognizingthe amount of electric energy charged into the electric double layercapacitor; and assist time calculating means for calculating an assisttime used as a time during which the motor can be operated continuouslyby the requested output on the basis of the requested output and theamount of electric energy charged into the electric double layercapacitor in an abruptly increasing state of the requested output inwhich the requested output exceeds a first output limit and anincreasing rate of the requested output exceeds a predeterminedreference increasing rate, the first output limit being determined inaccordance with a continuous output rating of the electric motor or themotor driving means, wherein when the requested output is not in theabruptly increasing state, the motor-target-output calculating meanssets the first output limit to the upper-limit target output, and whenthe requested output is in the abruptly increasing state, themotor-target-output calculating means sets a second output limit largerthan the first output limit to the upper-limit target output within theassist time, the second output limit being determined in accordance witha short-time output rating of the motor driving means or the electricmotor.
 2. The device according to claim 1, wherein themotor-target-output calculating means reduces the upper-limit targetoutput to the first output limit at a predetermined decreasing rateafter the assist time has passed.